Your phone should have options to specify how and when calls are diverted to your voicemail.
If your phone does not have menu options for this you can use the following codes to control the diverts.
You dial the following as if making a phone call
Divert All calls to voicemail
Divert on No Answer
Delay nn seconds: max 30 seconds, in 5 second increments
Divert on Unreachable
Divert on Busy
Please note: With 1pMobile, like all pay-as-you-go mobile services, it is not possible to divert calls to any other number other than to voicemail platform at +447953222222. This restriction is in place as there is a possibility of a breach of personal data if your service does not have sufficient credit for the divert to be applied (or your credit runs out during the diverted call). The caller would hear the announcement rather than you, the service holder.
If you require the facility to divert calls to another number then you will need to obtain a post-pay (contract) service from an alternative provider.