Bulk SMS in the UK means submitting the same message to many numbers through a commercial gateway, either as a list upload or as an API call per recipient. Three things decide whether it works: how fast you are allowed to send, what the sender ID is allowed to say, and whether you can prove the recipients agreed to hear from you. Everything else is template design.
This is what each of those involves, without the campaign language.
Throughput: what the numbers mean
Look for “how many SMS can I send per second” and you will find figures from one per second for a local number to “thousands per second” for an API. Both are true and neither is useful on its own, because throughput is a property of the route and the account, not of SMS.
What we will say specifically is this: submit no more than about ten messages per second, and pace the loop yourself. There is no machine-readable rate-limit header to obey, so a tight retry loop is the thing that gets an account throttled. If your volume needs more than that, it is a conversation about the route rather than a setting you can raise.
The practical consequence is that “bulk” is a queue, not a blast. A hundred thousand messages at ten
per second is nearly three hours of submission, which is fine if you planned for it and awkward if a
campaign has to be at a handset by nine in the morning. Plan from the queue length, and use the
delay parameter — up to 60 seconds — only for spread, not for scheduling a campaign.
Sender ID: what a UK network will accept
An alphanumeric sender ID can be up to 11 characters, using letters, digits and spaces. It has to be registered with us before traffic flows, typically within a working day, and it is checked against the reserved and restricted lists that UK operators maintain.
Two rules catch people out. First, some generic names are blocked by default — Alert, Bank,
Verify, Info, Support, Security, Update and similar — because they are exactly what a
phishing message would claim to be. A brand name is safer than a category name, which is also better
for the recipient. Second, protected brands and public bodies cannot be spoofed; using one usually
needs a letter of authorisation from whoever owns it. Our UK SMS rules page sets out
both, and the registration details we ask for.
Bulk campaigns are where these rules bite hardest, because the sender ID is the same across hundreds of thousands of messages: get it wrong and the whole campaign is rejected or substituted rather than a few messages.
Consent and opt-outs
Under PECR, marketing by text needs consent or the narrow soft opt-in for existing customers, and every
marketing message needs a working opt-out. The UK does not mandate the word STOP at network level the
way US carriers do — which is a good reason to use it anyway, because recipients expect it.
An opt-out that works has three parts, and most implementations have one:
- Receive it. The reply has to arrive somewhere you control, which means sending from a number that can receive, not an alphanumeric ID.
- Act on it immediately. Not at the next campaign, not in a weekly import. A suppression list is only a suppression list if the send path consults it.
- Keep it. Opt-outs outlive campaigns, imports and staff. Store them against the number in E.164, and treat a re-import of a customer list as the moment the list gets filtered, not the moment consent gets reset.
The legal detail, including the soft opt-in and the transactional distinction, is on UK SMS compliance; our anti-spam policy covers what we do with the sending side.
Personalisation without doubling the bill
The cheapest improvement to a bulk campaign is to stop it being one template with a first name bolted on, and the most expensive mistake is to bolt one on badly. Encoding decides the cost: a message that stays inside GSM 7-bit characters is 160 characters in one part, and every part is billed at 10.71 credits to a UK destination. A single character outside that alphabet — a curly apostrophe pasted from a word processor, an emoji, a ”£” is fine but a ”–” may not be — can switch the whole message to Unicode, drop the capacity to 70 characters, and turn one part into three.
Check the template with the fields filled in, not the placeholder version, and measure the parts before the campaign rather than after the invoice.
Receipts, and knowing what happened
Bulk traffic is where “we sent it” and “it arrived” diverge most, because the volume makes the
difference visible. Receipts arrive as they do for single messages — DELIVRD, EXPIRED, REJECTD,
UNDELIV — and they are the only honest source for campaign reporting. If a campaign shows a large
EXPIRED share, the cause is usually routing or an invalid handset rather than the copy.
Do not poll for status. The query operation is deprecated, and at bulk volumes it is the first thing that falls over.
A checklist that fits on one page
- Sender ID registered, non-generic, and checked before the campaign is built.
- Consent recorded, with the date and what the customer was told.
- Opt-outs received on a number that can receive, and consulted at send time.
- Template measured with real field values, in GSM characters, one or two parts at most.
- Queue sized against the throughput you actually have, not the throughput you hoped for.
- Receipts consumed and reconciled, because that is the only delivery figure worth reporting.
The bulk SMS service page covers the platform side, and coverage and rates has the per-destination figures.
