SIN 227 doesn't allow '+' in the number field. Wish I'd known this before trying to work out why it didn't appear...
The protocol used (SIP) to deliver phone calls to you has fields which provide the caller's number and the caller's "name" - although the name will probably not be received for calls from the PSTN. Where the name field received is the same as the number field, we will send to you the name field set to the "pretty format" of the caller's number - e.g. "020 xxxx xxxx", "07973 xxxxxx". If the name field is set to something different than the number then it will be passed on to you - in general this will only apply to calls from other AAISP VoIP customers, or from calls received using sip: URIs.
Your local hardware will display either the number, the name, or both (or neither if it has no display...). If you have an Analogue Telephone Adaptor set to sending caller display to your analogue phone(s) using BT's SIN 227 standard, then this standard also has fields for caller's number and caller's name and those fields will be populated.
For some examples, a SIP From header could be: