Hello @Carol202,
Is this a guest who has a confirmed reservation with you?
Their number should be on the confirmation message or email you received from Airbnb.
It will start with + so, instead of that + you dial the international access code of the country you are in at the moment.
If you are dialling from a mobile phone you can dial an actual + and then the number shown in the Airbnb confirmation message.
If they have given you a number in a message or email then you need to provide more information for people here to help.
Has the guest given just their national number or have they given the country code as well?
What country are you in?
What country is the guest's phone registered in?
Do not give the guest's number in any reply here as it will be removed by Aribnb.
Steve.