You have to go to 'your questions'
https://www.britishairways.com/trave...club/_gf/en_gb and then search for 'silver service centre' (it isn't exactly obvious....). First answer is 'Contacting your dedicated Silver Service Centre'
Your dedicated Silver Executive Club telephone number for all your bookings (both new and existing), BA Miles and Tier Points queries is 0800 xxx xxxx (opening hours 0600 to 2000, 7 days)
Membership queries and assistance is available from 0800-2000 hours, Monday to Friday and 0900-1700 hours on Saturday and Sunday.
To call from overseas, please dial + 44 191 xxx xxxx.
PM sent with real numbers.
Depending on your telephone provider you could be able to dial the 0800 number from abroad (but you will still be charged). Not sure if the geographical number is Silver-specific (the Gold number is the same).