The good news is, call diverts can be applied when you are outside the UK, free of charge. However, as you would be roaming, dependent on location and allowance, charges could be incurred for usage. 

The full list of available diverts is here: http://service.o2.co.uk/IQ/SRVS/CGI-BIN/WEBCGI.EXE?New,Kb=Companion,question=ref(User):str(Mobile),CASE=13448

Manage your costs by setting a Spend Control¹, through your my.plan dashboard. 

Go to: CONTROLS > SPEND CONTROLS > CALL LIMITS > UK/EU VOICE > [select user] > SET UP   

Alternatively, set an Ofcom Total Spend Cap². Go to: 

CONTROLS > SPEND CONTROLS > OFCOM TOTAL SPEND CAP > [select user] > SET UP

Need a bolt-on³, to provide reduced cost roaming? Go to:

PRODUCTS > VOICE > choose from the option:

• Daily Explorer (single day allowances)
• Executive Traveller (single day allowances for Exec.plan tariff subscribers)
• Global Traveller Minutes (calendar monthly allowances)

¹  available to Gold & Platinum account users and Essential account users via an upgrade  

²  available to new customers from 1st October 2018 and existing customer who renew from the same date  

³ account permissions may apply. Contact your account administrator.