Hi alb52547777alb52547777
There isn't an explicit 'prefix' field corresponding to the phone number which could allow you to update the prefix from 0 to 61 within a Workflow Activity.
You might have to change the phone numbers completely and that would be too inefficient since the query will have all the possible phone numbers (starting with 0) to be replaced with phone numbers starting with 61.
However you could consider the TON and NPI Settings in case the SMS Connection is being established with vendor through Extended Generic SMPP, see below Screen Shot:

NPI (Numbering Plan Identification) and TON (Type of Number) combination might allow the phone number prefix to be 0 on your side and 61 on vendor's side. For '0', TON value would be '2' (National) and for '61', TON value would be '1' (International).
The NPI value becomes '1' (ISDN/telephone numbering plan) for numbers starting with '61' and '8' for numbers starting with '0'.
Considering you are on v7, I am assuming that you might be using Extended Generic SMPP Connector only.
If so, then you could try this combination once.
For reference, below are the standard NPI and TON values:
TON:
Unknown = 0
International = 1
National = 2
Network Specific = 3
Subscriber Number = 4
Alphanumeric = 5
Abbreviated = 6
NPI:
Unknown = 0
ISDN/telephone numbering plan (E163/E164) = 1
Data numbering plan (X.121) = 3
Telex numbering plan (F.69) = 4
Land Mobile (E.212) =6
National numbering plan = 8
Private numbering plan = 9
ERMES numbering plan (ETSI DE/PS 3 01-3) = 10
Internet (IP) = 13
WAP Client Id (to be defined by WAP Forum) = 18
Hope this helps!