Auto Mapper convert from string to Int

The problem you may face is when it is unable to parse the string, one option would be to use a ResolveUsing:

Mapper.CreateMap<SourceClass, DestinationClass>() 
.ForMember(dest => dest.intphoneno, opts => opts.ResolveUsing(src =>
                double.TryParse(src.strphoneno, out var phoneNo) ? phoneNo : default(double)));

Mapper.CreateMap<SourceClass, DestinationClass>() 
    .ForMember(dest => dest.IntphoneNo, 
        opt => opt.MapFrom(src => int.Parse(src.Stringphoneno)));

Here is some sample working code using the map described

class SourceClass
{
    public string Stringphoneno { get; set; }
}

class DestinationClass
{
    public int IntphoneNo { get; set; }
}

var source = new SourceClass {Stringphoneno = "8675309"};
var destination = Mapper.Map<SourceClass, DestinationClass>(source);

Console.WriteLine(destination.IntphoneNo); //8675309