A corporate account? Is it possible that your company's Exchange Server administrator has set a rule in place to limit characters? My signature block includes my full name, title, company name & address, telephone and fax numbers, email address, web site address, company logo (as a jpg image), and a quite long privacy/disclaimer statement. In total, probably 500+ characters.
So, unless you want to have a conversation with your email administrator, the only other thought would be to make use of a "URL shortener" service such as
https://bitly.com/
PS: what email client are you using on your mobile device?
