1. Verification would be optional. It would be an increase of trust and maybe change in forum privileges.
2. phpBB could make some profit from provisions from the link between phpBB and network paid by forum admins. Could you send link to potential costs?
1. It would involve having a physical phone number which require SIM card. Does netbook have one? How difficult is to fake the number?
2. Where do I start with the Jabber. Seems complicated.
1. Verification would be optional aiming at increase user trust, not only confirming identity.
Is that a service where users can purchase a membership?
After giving bit more thought to the idea I am now not sure about it as sharing mobile number might be too private for users especially for small forums (Have no experience in that matter).